1. Field of the Invention
The present invention relates to movable micro-devices such as variable micro-capacitors and micro sensing devices which are manufactured by micromachining technology and have tiny movable parts.
2. Description of the Related Art
In recent years, movable micro-devices which are manufactured by micromachining technology have been drawing attention in various technical fields, and efforts have been made for application of these devices which have a microstructure. Micro devices include such devices as variable micro-capacitors and micro sensing devices which have tiny movable parts or oscillating parts. Movable micro-devices are disclosed in the following Patent Documents 1 through 3, for example.    Patent Document 1: JP-A-2002-373829    Patent Document 2: JP-A-2004-505788    Patent Document 3: U.S. Pat. No. 5,959,516
FIG. 16 through FIG. 19 show a variable micro-capacitor X4 as an example of conventional movable micro-device. FIG. 16 is a plan view of the variable micro-capacitor X4. FIG. 17 is another plan view of the variable micro-capacitor X4. FIG. 18 and FIG. 19 are sectional views taken along lines XVIII-XVIII and XIX-XIX respectively, in FIG. 16.
The variable micro-capacitor X4 includes a movable part 40, a frame 50 and a pair of connecting parts 60. The variable micro-capacitor X4 is manufactured by micromachining technology such as MEMS technology, from a so called SOI (silicon on insulator) material substrate. The material substrate has a laminate structure including a first and a second silicon layer, and an insulation layer between the silicon layers. Each of the silicon layers is doped with impurity and has a predetermined level of electrical conductivity. FIG. 16 is a plan view intended primarily to clarify a structure formed from the first silicon layer. For the sake of clarification, those portions formed from the first silicon layer are hatched in FIG. 16. In FIG. 17, portions formed from the second silicon layer are hatched.
The movable part 40 is formed entirely from the first silicon layer, has a movable main portion 41 and comb-teeth electrodes 42, 43, and is swingable or rotatable with respect to the frame 50. The comb-teeth electrode 42 is provided by a plurality of electrode teeth 42a which extend from the movable main portion 41. The comb-teeth electrode 43 is provided by a plurality of electrode teeth 43a which extend from the movable main portion 41.
The frame 50 includes a frame main portion 51 and comb-teeth electrodes 52, 53. The frame main portion 51, which has a laminate structure including the above-described first and the second silicon layers and the insulation layer between the silicon layers, surrounds the movable part 40. The comb-teeth electrodes 52 is provided by a plurality of electrode teeth 52a which extends from the frame main portion 51. The comb-teeth electrode 53 is provided by a plurality of electrode teeth 53a which extends from the frame main portion 51. The frame main portion 51 is formed with a gap 51a at a predetermined location in the portion which is formed from the first and the second silicon layers. Because of the gap 51a and the insulation layer between the silicon layers, the comb-teeth electrodes 42, 43 are electrically separated from the comb-teeth electrodes 52, 53, and the comb-teeth electrode 52 is electrically separated from the comb-teeth electrode 53 in the frame 50.
The comb-teeth electrode 42 of the movable part 40 and the comb-teeth electrode 52 of the frame 50 constitute a pair of capacitor electrodes in the variable micro-capacitor X4. In an initial position, electrode teeth 42a of the comb-teeth electrode 42 and electrode teeth 52a of the comb-teeth electrodes 52 have their side surfaces opposed to each other.
The comb-teeth electrode 43 of the movable part 40 and the comb-teeth electrode 53 of the frame 50 constitute a pair of driver electrodes in the variable micro-capacitor X4. The comb-teeth electrode 43 is formed from the first silicon layer whereas the comb-teeth electrode 53 is formed from the second silicon layer.
Each connecting part 60 connects the movable part 40 and the frame 50. The pair of connecting parts 60 define an axis A4 for rotational displacement of the movable part 40 with respect to the frame 50. Also, each connecting part 60 electrically connects part of the frame main portion 51 with the movable part 40. The movable part 40 which has the comb-teeth electrodes 42, 43 is grounded via the connecting part 60.
FIG. 20 and FIG. 21 show a method for manufacturing the variable micro-capacitor X4. FIG. 20 and FIG. 21 show a sectional view in series, following steps of forming the movable main portion 41, the comb-teeth electrodes 42, 43, the frame main portion 51, the comb-teeth electrodes 52, 53, and the connecting part 60. The section shown in the views is a conceptual composite representing various portions from different sections of the material substrate (wafer) to which micromachining is performed.
In the manufacture of the variable capacitor X4, first, a material substrate 300 as shown in FIG. 20(a) is prepared. The material substrate 300 is a so called SOI, and has a laminate structure including silicon layers 301, 302, and an insulation layer 303 between the silicon layers.
Next, as shown in FIG. 20(b), a resist pattern 304 is formed on the silicon layer 301. The resist pattern 304 provides a pattern for portions to be formed from the silicon layer 301 in the variable micro-capacitor X4.
Next, as shown in FIG. 20(c), a resist pattern 305 is formed on the silicon layer 302. The resist pattern 305 provides a pattern for portions to be formed from the silicon layer 302 in the variable micro-capacitor X4.
Next, as shown in FIG. 21(a), anisotropic dry etching is performed to the silicon layer 301, using the resist pattern 304 as a mask, whereby formation is made for the movable main portion 41, the comb-teeth electrodes 42 or electrode teeth 42a, the comb-teeth electrode 43 or electrode teeth 43a, part of the frame main portion 51, the comb-teeth electrodes 52 or electrode teeth 52a and the connecting part 60.
Next, as shown in FIG. 21(b), anisotropic dry etching is performed to the silicon layer 302, using the resist pattern 305 as a mask, whereby formation is made for part of the frame main portion 51 and the comb-teeth electrode 53 or electrode teeth 53a. 
Next, as shown in FIG. 21(c), resist patterns 304, 305 are removed, and exposed portions of the insulation layer 303 are removed. Through these steps, it is possible to manufacture the variable micro-capacitor X4.
In the variable micro-capacitor X4, it is possible to rock or rotationally displace the movable part 40 about the axis A4 as necessary, by applying a predetermined electric potential to the comb-teeth electrode 53. As the predetermined electric potential is applied to the comb-teeth electrode 53, a predetermined electrostatic attraction is generated between the comb-teeth electrodes 43, 53 (Note that the comb-teeth electrode 43 is grounded in the present example), and therefore the comb-teeth electrode 43 is drawn toward the comb-teeth electrode 53. As a result, the movable part 40 rocks about the axis A4, and makes a rotational displacement to an angle at which the electrostatic attraction is counterbalanced by a sum of torsional stresses in the connecting parts 60. The amount of rotational displacement caused by such a rocking operation is adjustable through adjustment of the electric potential applied to the comb-teeth electrode 53 (The adjustment on the amount of rotational displacement may be made by controlling the electric potential difference between the comb-teeth electrodes 43, 53, without grounding the comb-teeth electrode 43). Through the adjustment on the amount of rotational displacement, it is possible to adjust the amount of opposed area between the comb-teeth electrodes 42, 52 (the area of side surfaces via which the electrode teeth 42a and the electrode teeth 52a oppose to each other), and therefore it is possible to adjust the electrostatic capacity between the comb-teeth electrodes 42, 52 which serve as the capacitor electrodes. On the other hand, if the electrostatic attraction between the comb-teeth electrodes 43, 53 is removed, each connecting part 60 releases the torsional stresses held therein, to return to its natural state, allowing the movable part 40 or comb-teeth electrodes 42 to return to its initial position.
Generally, a capacitor device whose capacitor electrodes have a small resistance tends to have a high Q-value in the capacitor device. However, in the conventional variable micro-capacitor X4 which is manufactured by the above-described method from an SOI substrate, the pair of capacitor electrodes (comb-teeth electrodes 42, 52) are made of an electroconductive silicon material, and the electroconductive silicon material generally has a higher resistivity than e.g. a metal material. For this reason, it is sometimes impossible to achieve a sufficiently high Q-value in the variable micro-capacitor X4 (movable micro-device)